JAGMAN1

'04 CTT, runs and drives great, but after being parked for 5-7 days the rear end begins to sag visibly. After startup, and prior to driving off, the rear end raises back to normal height. I've take some measurements and both left and right rear are settling evenly.

I'm about to start troubleshooting this and wanted to know if anyone can point me in the correct direction.

I'll check the whole system over, but I find it interesting the rear end is settling evenly and not to one side or the other. So, I don't think it can be a leaking air spring, as this would cause only one side to drop. Unless of course the left and right sides failed simultaneously.

I've scanned with Durametric and have zero codes. I drive the car 1-2 times per week, and that's why I'm able to confirm the rate of the leak, and am fairly confident of my description of the behavior.

Thanks!

JAGMAN1

Does anyone have the service manual pdf for the air suspension? I thought I had it in my library, but apparently I don't.

Sniffer

It does sound like a leak but not at an individual shock for the reasons you say

Is there anything "common" to both (air supply line/separate header tank/etc etc)?

JAGMAN1

So last night I replaced the L & R rear pressure holding valves. They are the brass fittings that the air supply lines plug into, and are located at the top of the struts. It's a pretty easy and straightforward job.

Did it work? I don't know yet.

I took ride height measurements last night and again this morning. The measurements looked good. But the slow leak manifests a drop in ride height over a few days, so I will update after a bit more time has passed.

AGARubberDuck

I can't speak for the rear air springs specifically, but the front are definitely in series with each other. I had one blow out and the entire front came down, not just one corner. Replaced that single spring(to limp the truck home) and the system was back to normal.

I'd guess a small leak either around one of the brass fittings would be a relatively easy thing to check/remove&replace. From putting one together I can tell you another spot I'd suspect is the orings between the spring and the shock. Theyvfit kinda tight and require petroleum jelly to make the seal air-tight. That's a pretty large job to resolve though, and I wouldn't if it isn't throwing a code.

JAGMAN1

That's interesting. A few weeks ago when I checked the system for leaks, and traced the air supply lines I learned about how the system is designed. It seems that the air compressor feeds the distribution block. Then the accumulators and each air spring connect individually to the distribution block. The arrangement suggests that each spring has an independent air supply, and should fail singularly.

In the end. I've been driving the car the last few days and haven't given it the chance to leak down to see if my repair was successful.

In the meantime, I've been trying to understand how the pressure holding valves work. At present this escapes me. Perhaps someone can shed some light. My observations:
The pressure holding valves screw into the air-spring on one end, and have the air supply line on the other end. During replacement, I first removed the supply line, and some air hissed as it was removed (the air that leaked out was small in volume and my estimation was the residual pressure between the distribution block and the spring). But, the strut remained pressurized and the car at normal ride height. It wasn't until I removed the pressure holding valve from the air-spring that the ride height collapsed.

I don't understand how this valve can be a two way valve without any electrical actuator. It's pressurized on both sides, and seems to let air in or out depending on???

AGARubberDuck

I suppose I may have misjudged the situation. When my air spring blew out, it took with it the drop link for the ride-height sensor (between sensor and upper control arm). It's possible the sensor arm was in such a position telling the passenger side to remain low. Just assumed the front was in series by its behavior.

JAGMAN1

Ok. Final report. It is fixed. Replacing the rear air suspension holding valves has solved the problem. No more droopy rear end.

So, either they were both leaking, or if one leaks the rear drops evenly due to the sway bar.

John Welch.

Another data point, my right rear sags overnight. Looks like I will need to do this repair.

4ess

Air pump will eventually fail or the spring will burst. Car will be barely driveable at that point until fixed.